A solar cell is formed of two semiconductor layers in facing contact with each other at a semiconductor junction. When illuminated by the sun or other light source, the solar cell produces a voltage between the semiconductor layers. Advanced solar cells may include more than two semiconductor layers and their respective pairwise semiconductor junctions. The various pairs of semiconductor layers of the advanced solar cells form subcells, with each subcell tuned to a specific spectral component of the sun to maximize the power output. The voltage and electrical current output of the solar cell are limited by the materials of construction and the surface area of the solar cell. Most commonly, a number of subcells are electrically interconnected in series to form a solar cell structure that produces higher voltages than are possible with the single-junction solar cell. Such multijunction solar cell structures with up to three subcells are now used in both space and terrestrial applications. These solar cell structures work well when all of the subcells absorb about the same photon flux so that they all produce the same electrical current.
When single-junction or multijunction solar cells form a circuit of serially connected devices, and one of the solar cells in the circuit is shaded while the others remain fully illuminated, the shaded solar cell is subjected to a reverse-bias condition by the continuing voltage and current output of the remaining unshaded solar cells. Fortunately, each solar cell may be protected against the potential damage arising during the reverse-bias condition by an electrically parallel diode that does not pass current when the solar cell is not reverse biased, but passes the impressed current when the solar cell is reverse biased. The diode thus protects the individual cell against reverse-bias damage.
A number of diode configurations are in use and are operable, but each has its drawbacks. In one configuration, a discrete diode is bonded to the backside of the solar cell and interconnected to the semiconductor layers of the solar cell with leads. This approach requires the bonding of the interconnection taps and the leads, a time-consuming process when a large number of solar cells are present in the solar cell circuit. In another configuration, the diode is grown onto the front surface of the solar cell as part of the deposition process and then interconnected to the next solar cell in series. The available approaches are complex and cause assembly difficulties as well as reduced production yields and reduced solar cell efficiencies. In yet another configuration, the diode is also grown into the front surface of the solar cell and interconnected with discrete or lithographic techniques. This approach is also complex, and has reduced production yields and reduced solar cell efficiency.
There is a need for an improved approach to the protection of solar cells against reverse-bias damage. The present invention fulfills this need, and further provides improved operating efficiency and other related advantages.